Due to excellent chemical and thermal stability properties of fluoropolymers, they are used in many areas of technology. However, a fluoropolymer having a hydrophobic surface has proven to be a disadvantage in certain applications such as liquid filtration. Pure fluoropolymers cannot be used for separating disperse particles in water because they cannot be wetted with water. In order to use them for separating substances from aqueous media, a number of methods have been developed to make the surface of the fluoropolymer wettable with water, thus making them hydrophilic.
Numerous proposals have been made for imparting a hydrophilic character to a fluoropolymer surface. For example, fluoropolymer membranes have been wetted with aqueous alcohol or ketone solutions. The hydrophilic properties for these membranes are maintained only in the wet state, thus a permanent hydrophilic property is not achieved. According to U.S. Pat. Nos. 4,525,374 and 4,839,298, the fluoropolymer membrane is impregnated with a mixture of solvent and surfactant. According to European Patent 175,322, a membrane is impregnated with a hydrophilic propylene glycol monofatty acid ester. The disadvantage of these methods of treatment is that the surfactant is washed out during the use of the membrane, especially at high temperatures. It is also known from U.S. Pat. Nos. 4,298,002, and 4,908,236 that a microporous fluoropolymer membrane can be impregnated with a hydrophilic monomer and then polymerized. U.S. Pat. Nos. 4,113,912, 4,917,773, 4,618,533 and 4,506,035 describe impregnation of the fluoropolymer membrane with a hydrophilic monomer. Following impregnation, graft polymerization, crosslinking, a plasma treatment or a treatment with high energy radiation is performed for the purpose of fixation. However, it has been found that these methods lead to an irregular finish of the membrane and to an irregular crosslinking. In addition there is also the danger that the pores of a fluoropolymer membrane can become blocked. In Polymer Preprints 31 (1990) 1, M. S. Shoichet/T. J. McCarthy, U.S. Pat. No. 3,390,067 and European Patent 245,000, chemical pretreatment of the fluoropolymer surface with reducing reagents and then graft copolymerization are described. J. Appl. Polym. Sci., 26 (1990) 2637, E.-S. A. Hegazy, N. N. Taher, A. R. Ebaid and U.S. Pat. No. 4,734, 112 describe pretreatment of a fluoropolymer membrane with plasma, laser or by glow discharge. This is followed by a chemical reaction with a hydrophilic substance. These methods lead to a loss of mechanical stability of the membrane and to a nonhomogeneous modification of the area near the surface. European Patent Nos. 407,900 and 456,939 describe modification of a fluoropolymer membrane with a hydrophilic polymer and then chemical reaction of the hydrophilic groups. This method has the disadvantage that the membrane has a reduced solvent stability and there is a loss of the hydrophilic character at elevated temperatures due to reorientation effects and thermal instability of the functional groups. According to European Patent No. 408,378 and U.S. Pat. No. 4,113,912, a fluoropolymer membrane is rendered hydrophilic by treating it with a hydrophilic polymer and then completing this hydrophilic polymer with a complexing agent. Although hydrophilic membranes produced in this way have a stabilized modification, binding of the hydrophilic components is inadequate and there is the danger of inactivation due to loss of the finish. European Patent No. 436,720 describes the use of polyelectrolytes consisting of polycationic and polyanionic compounds that can impart hydrophilic properties to certain substrates. However, it has been found that these complexes yield completely inadequate adhesion to fluoropolymer membranes. Polyelectrolyte complexes are also disclosed in East German Patent Nos. 280,257 and 280,258 which can induce a cationic modification of substrates. Here again, the adhesion is inadequate.
In summary, none of the references teach a permanent hydrophilic modification for fluoropolymers that also provides for adequate hydrophilic properties, chemical resistance, mechanical strength, water permeability and permanence.
There is a need for a permanent hydrophilic modification for fluoropolymers, especially fluoropolymer membranes that have an increased permanence and whereby at the same time the normally desirable properties such as chemical resistance and mechanical strength of the fluoropolymers are maintained.